This invention relates to the genetic control of growth and/or development of plants and the cloning and expression of genes involved therein. More particularly, the invention relates to the cloning and expression of the GAI gene of Arabidopsis thaliana, and homologues from other species, and use of the genes in plants.
An understanding of the genetic mechanisms which influence growth and development of plants, including flowering, provides a means for altering the characteristics of a target plant. Species for which manipulation of growth and/or development characteristics may be advantageous includes all crops, with important examples being the cereals, rice and maize, probably the most agronomically important in warmer climatic zones, and wheat, barley, oats and rye in more temperate climates. Important crops for seed products are oil seed rape and canola, sugar beet, maize, sunflower, soyabean and sorghum. Many crops which are harvested for their roots are, of course, grown annually from seed and the production of seed of any kind is very dependent upon the ability of the plant to flower, to be pollinated and to set seed. In horticulture, control of the timing of growth and development, including flowering, is important. Horticultural plants whose flowering may be controlled include lettuce, endive and vegetable brassicas including cabbage, broccoli and cauliflower, and carnations and geraniums. Dwarf plants on the one hand and over-size, taller plants on the other may be advantageous and/or desirable in various horticultural and agricultural contexts.
Arabidopsis thaliana is a favourite of plant geneticists as a model organism. Because it has a small, well-characterized genome, is relatively easily transformed and regenerated and has a rapid growing cycle, Arabidopsis is an ideal model plant in which to study growth and development and its control.
Many plant growth and developmental processes are regulated by specific members of a family of tetracyclic diterpenoid growth factors known as gibberellins (GA)1. The gai mutation of Arabidopsis confers a dwarf phenotype and a dramatic reduction in GA-responsiveness2-9. Here we report the molecular cloning of gai via Ds transposon mutagenesis.
The phenotype conferred by the Ds insertion allele confirms that gai is a gain-of-function mutation, and that the wild-type allele (GAI) is dispensable5,6. GAI encodes a novel polypeptide (GAI) of 532 amino acid residues, of which a 17 amino acid domain is missing in the gai mutant polypeptide. This result is consistent with GAI acting as a plant growth repressor whose activity is antagonized by GA. Though we are not to be bound by any particular theory, gai may repress growth constitutively because it lacks the domain that interacts with the GA signal. Thus according to this model GA regulates plant growth by de-repression.
gai is a dominant, gain-of-function mutation, which confers a dark-green, dwarf phenotype, and interferes with GA reception or subsequent signal-transduction2-9. Dominant mutations conferring similar phenotypes are known in other plant species, including maize10-12 and wheat13. The latter are especially important because they are the basis of the high-yielding, semi-dwarf wheat varieties of the xe2x80x98green revolutionxe2x80x9914. The increased yield of these varieties is due to an increased grain production per ear, and superior straw strength. The shorter, stronger straw greatly reduces the losses resulting from lodging, that is flattening of standing wheat plants by rain/wind. We set out to clone gai from Arabidopsis because of its importance to the understanding of GA signal-transduction, and because of the potential for use of GA-insensitivity in the development of wheat and other crops such as oil-seed rape and rice which may show improvement as great as that already seen in wheat.
According to a first aspect of the present invention there is provided a nucleic acid molecule comprising a nucleotide sequence encoding a polypeptide with GAI function. The term xe2x80x9cGAI functionxe2x80x9d indicates ability to influence the phenotype of a plant like the GAI gene of Arabidopsis thaliana. xe2x80x9cGAI functionxe2x80x9d may be observed phenotypically in a plant as inhibition, suppression, repression or reduction of plant growth which inhibition, suppression, repression or reduction is antagonised by GA. GAI expression tends to confer a dwarf phenotype on a plant which is antagonised by GA. Overexpression in a plant from a nucleotide sequence encoding a polypeptide with GAI function may be used to confer a dwarf phenotype on a plant which is correctable by treatment with GA.
Also according to an aspect of the present invention there is provided a nucleic acid molecule comprising a nucleotide sequence encoding a polypeptide with ability to confer a gai mutant phenotype upon expression. gai mutant plants are dwarfed compared with wild-type, the dwarfing being GA-insensitive.
By gibberellin or GA is meant a diterpenoid molecule with the basic carbon-ring structure shown in FIG. 1 and possessing biological activity, i.e. we refer to biologically active gibberellins.
Biological activity may be defined by one or more of stimulation of cell elongation, leaf senescence or elicitation of the cereal aleurone xcex1-amylase response. There are many standard assays available in the art, a positive result in any one or more of which signals a test gibberellin as biologically active28,29,30.
Assays available in the art include the lettuce hypocotyl assay, cucumber hypocotyl assay, and oat first leaf assay, all of which determine biological activity on the basis of ability of an applied gibberellin to cause elongation of the respective tissue. Preferred assays are those in which the test composition is applied to a gibberellin-deficient plant. Such preferred assays include treatment of dwarf GA-deficient Arabidopsis to determine growth, the dwarf pea assay, in which internode elongation is determined, the Tan-ginbozu dwarf rice assay, in which elongation of leaf sheath is determined, and the d5-maize assay, also in which elongation of leaf sheath is determined. The elongation-bioassays measure the effects of general cell elongation in the respective organs and are not restricted to particular cell types.
Further available assays include the dock (Rumex) leaf senescence assay and the cereal aleurone xcex1-amylase assay. Aleurone cells which surround the endosperm in grain secrete xcex1-amylase on germination, which digests starch to produce sugars then used by the growing plant. The enzyme production is controlled by GA. Isolated aleurone cells given biologically active GA secrete xcex1-amylase whose activity can then be assayed, for example by measurement of degradation of starch.
Structural features important for high biological activity (exhibited by GA1, GA2, GA4 and GA7) are a carboxyl group on C-6 of B-ring; C-19, C-10 lactone; and xcex2-hydroxylation at C-3. xcex2-hydroxylation at C-2 causes inactivity (exhibited by GA8, GA29, GA34 and GA51). gai mutants do not respond to GA treatment, e.g. treatment with GA1, GA3 or GA4.
Treatment with GA is preferably by spraying with aqueous solution, for example spraying with 10xe2x88x924M GA3 or GA4 in aqueous solution, perhaps weekly or more frequently, and may be by placing droplets on plants rather than spraying. GA may be applied dissolved in an organic solvent such as ethanol or acetone, because it is more soluble in these than in water, but this is not preferred because these solvents have a tendency to damage plants. If an organic solvent is to be used, suitable formulations include 24 xcex7l of 0.6, 4.0 or 300 mM GA3 or GA4 dissolved in 80% ethanol. Plants, e.g. Arabidopsis, may be grown on a medium containing GA, such as tissue culture medium (GM) solidified with agar and containing supplementary GA.
Nucleic acid according to the present invention may have the sequence of a wild-type GAI gene of Arabidopsis thaliana, or be a mutant, derivative, variant or allele of the sequence provided. Preferred mutants, derivatives, variants and alleles are those which encode a protein which retains a functional characteristic of the protein encoded by the wild-type gene, especially the ability for plant growth inhibition, which inhibition is antagonised by GA, or ability to confer on a plant one or more other characteristics responsive to GA treatment of the plant. Other preferred mutants, derivatives, variants and alleles encode a protein which confers a gai mutant phenotype, that is to say reduced plant growth which reduction is insensitive to GA, i.e. not overcome by GA treatment. Changes to a sequence, to produce a mutant, variant or derivative, may be by one or more of addition, insertion, deletion or substitution of one or more nucleotides in the nucleic acid, leading to the addition, insertion, deletion or substitution of one or more amino acids in the encoded polypeptide. Of course, changes to the nucleic acid which make no difference to the encoded amino acid sequence are included.
A preferred nucleotide sequence for a GAI gene is one which encodes amino acid sequence shown in FIG. 4, especially a coding sequence shown in FIG. 3. A preferred gai mutant lacks part or all of the 17 amino acid sequence underlined in FIG. 4.
The present invention also provides a nucleic acid construct or vector which comprises nucleic acid with any one of the provided sequences, preferably a construct or vector from which polypeptide encoded by the nucleic acid sequence can be expressed. The construct or vector is preferably suitable for transformation into a plant cell. The invention further encompasses a host cell transformed with such a construct or vector, especially a plant cell. Thus, a host cell, such as a plant cell, comprising nucleic acid according to the present invention is provided. Within the cell, the nucleic acid may be incorporated within the chromosome. There may be more than one heterologous nucleotide sequence per haploid genome. This, for example, enables increased expression of the gene product compared with endogenous levels, as discussed below.
A construct or vector comprising nucleic acid according to the present invention need not include a promoter or other regulatory sequence, particularly if the vector is to be used to introduce the nucleic acid into cells for recombination into the genome. However, in one aspect the present invention provides a nucleic acid construct comprising a GAI or gai coding sequence (which includes homologues from other than Arabidopsis thaliana) joined to a regulatory sequence for control of expression, the regulatory sequence being other than that naturally fused to the coding sequence and preferably of or derived from another gene.
Nucleic acid molecules and vectors according to the present invention may be as an isolate, provided isolated from their natural environment, in substantially pure or homogeneous form, or free or substantially free of nucleic acid or genes of the species of interest or origin other than the sequence encoding a polypeptide able to influence growth and/or development, which may include flowering, eg in Arabidopsis thaliana nucleic acid other than the GAI coding sequence. The term xe2x80x9cnucleic acid isolatexe2x80x9d encompasses wholly or partially synthetic nucleic acid.
Nucleic acid may of course be double- or single-stranded, cDNA or genomic DNA, RNA, wholly or partially synthetic, as appropriate. Of course, where nucleic acid according to the invention includes RNA, reference to the sequence shown should be construed as reference to the RNA equivalent, with U substituted for T.
The present invention also encompasses the expression product of any of the nucleic acid sequences disclosed and methods of making the expression product by expression from encoding nucleic acid therefor under suitable conditions in suitable host cells. Those skilled in the art are well able to construct vectors and design protocols for expression and recovery of products of recombinant gene expression. Suitable vectors can be chosen or constructed, containing appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate. For further details see, for example, Molecular Cloning: a Laboratory Manual: 2nd edition, Sambrook et al, 1989, Cold Spring Harbor Laboratory Press. Transformation procedures depend on the host used, but are well known. Many known techniques and protocols for manipulation of nucleic acid, for example in preparation of nucleic acid constructs, mutagenesis, sequencing, introduction of DNA into cells and gene expression, and analysis of proteins, are described in detail in Protocols in Molecular Biology, Second Edition, Ausubel et al. eds., John Wiley and Sons, 1992. Specific procedures and vectors previously used with wide success upon plants are described by Bevan, Nucl. Acids Res. (1984) 12, 8711-8721), and Guerineau and Mullineaux, (1993) Plant transformation and expression vectors. In: Plant Molecular Biology Labfax (Croy RRD ed) Oxford, BIOS Scientific Publishers, pp 121-148. The disclosures of Sambrook et al. and Ausubel et al. and all other documents mentioned herein are incorporated herein by reference.
Since the GAI amino acid sequence of Arabidopsis shown in FIG. 4 includes 5 consecutive histidines close to its N-terminus, substantial purification of GAI or gai may be achieved using Ni-NTA resin available from QIAGEN Inc. (USA) and DIAGEN GmbH (Germany). See Janknecht et al31 and EP-A-0253303 and EP-A-0282042. Ni-NTA resin has high affinity for proteins wiht consecutive histidines close to the N- or C-terminus of the protein and so may be used to purifiy GAI or gai proteins from plants, plant parts or extracts or from recombinant organisms such as yeast or bacteria, e.g. E. coli, expressing the protein.
Purified GAI protein, e.g. produced recombinantly by expression from encoding nucleic acid therefor, may be used to raise antibodies employing techniques which are standard in the art. Antibodies and polypeptides comprising antigen-binding fragments of antibodies may be used in identifying homologues from other species as discussed further below.
Methods of producing antibodies include immunising a mammal (eg human, mouse, rat, rabbit, horse, goat, sheep or monkey) with the protein or a fragment thereof. Antibodies may be obtained from immunised animals using any of a variety of techniques known in the art, and might be screened, preferably using binding of antibody to antigen of interest. For instance, Western blotting techniques or immunoprecipitation may be used (Armitage et al, 1992, Nature 357: 80-82). Antibodies may be polyclonal or monoclonal.
As an alternative or supplement to immunising a mammal, antibodies with appropriate binding specificty may be obtained from a recombinantly produced library of expressed immunoglobulin variable domains, eg using lambda bacteriophage or filamentous bacteriophage which display functional immunoglobulin binding domains on their surfaces; for instance see WO92/01047.
Antibodies raised to a GAI, or gai, polypeptide can be used in the identification and/or isolation of homologous polypeptides, and then the encoding genes. Thus, the present invention provides a method of identifying or isolating a polypeptide with GAI function or ability to confer a gai mutant phenotype, comprising screening candidate polypeptides with a polypeptide comprising the antigen-binding domain of an antibody (for example whole antibody or a fragment thereof) which is able to bind an Arabidopsis GAI or gai polypeptide, or preferably has binding specificity for such a polypeptide, such as having the amino acid sequence shown in FIG. 4.
Candidate polypeptides for screening may for instance be the products of an expression library created using nucleic acid derived from an plant of interest, or may be the product of a purification process from a natural source.
A polypeptide found to bind the antibody may be isolated and then may be subject to amino acid sequencing. Any suitable technique may be used to sequence the polypeptide either wholly or partially (for instance a fragment of the polypeptide may be sequenced). Amino acid sequence information may be used in obtaining nucleic acid encoding the polypeptide, for instance by designing one or more oligonucleotides (e.g. a degenerate pool of oligonucleotides) for use as probes or primers in hybridisation to candidate nucleic acid, as discussed further below.
A further aspect of the present invention provides a method of identifying and cloning GAI homologues from plant species other than Arabidopsis thaliana which method employs a nucleotide sequence derived from that shown in FIG. 3. Sequences derived from these may themselves be used in identifying and in cloning other sequences. The nucleotide sequence information provided herein, or any part thereof, may be used in a data-base search to find homologous sequences, expression products of which can be tested for GAI function. Alternatively, nucleic acid libraries may be screened using techniques well known to those skilled in the art and homologous sequences thereby identified then tested.
For instance, the present invention also provides a method of identifying and/or isolating a GAI or gai homologue gene, comprising probing candidate (or xe2x80x9ctargetxe2x80x9d) nucleic acid with nucleic acid which encodes a polypeptide with GAI function or a fragment or mutant, derivative or allele thereof. The candidate nucleic acid (which may be, for instance, cDNA or genomic DNA) may be derived from any cell or organism which may contain or is suspected of containing nucleic acid encoding such a homologue.
In a preferred embodiment of this aspect of the present invention, the nucleic acid used for probing of candidate nucleic acid encodes an amino acid sequence shown in FIG. 4, a sequence complementary to a coding sequence, or a fragment of any of these, most preferably comprising a nucleotide sequence shown in FIG. 3.
Alternatively, as discussed, a probe may be designed using amino acid sequence information obtained by sequencing a polypeptide identified as being able to be bound by an antigen-binding domain of an antibody which is able to bind a GAI or gai polypeptide such as one with the amino acid sequence shown in FIG. 4.
Preferred conditions for probing are those which are stringent enough for there to be a simple pattern with a small number of hybridizations identified as positive which can be investigated further. It is well known in the art to increase stringency of hybridisation gradually until only a few positive clones remain.
As an alternative to probing, though still employing nucleic acid hybridisation, oligonucleotides designed to amplify DNA sequences from GAI genes may be used in PCR or other methods involving amplification of nucleic acid, using routine procedures. See for instance xe2x80x9cPCR protocols; A Guide to Methods and Applicationsxe2x80x9d, Eds. Innis et al, 1990, Academic Press, New York.
Preferred amino acid sequences suitable for use in the design of probes or PCR primers are sequences conserved (completely, substantially or partly) between GAI genes.
On the basis of amino acid sequence information, oligonucleotide probes or primers may be designed, taking into account the degeneracy of the genetic code, and, where appropriate, codon usage of the organism from the candidate nucleic acid is derived.
The present invention also extends to nucleic acid encoding a GAI homologue obtained using a nucleotide sequence derived from that shown in FIG. 3.
Also included within the scope of the present invention are nucleic acid molecules which encode amino acid sequences which are homologues of the polypeptide encoded by GAI of Arabidopsis thaliana. A homologue may be from a species other than Arabidopsis thaliana. 
Homology may be at the nucleotide sequence and/or amino acid sequence level. Preferably, the nucleic acid and/or amino acid sequence shares homology with the sequence encoded by the nucleotide sequence of FIG. 3, preferably at least about 50%, or 60%, or 70%, or 80% homology, most preferably at least 90% or 95% homology. Nuleic acid encoding such a polypeptide may preferably share with the Arabidopsis thaliana GAI gene the ability to confer a particular phenotype on expression in a plant, preferably a phenotype which is GA responsive (i.e. there is a change in a characteristic of the plant on treatment with GA), such as the ability to inhibit plant growth where the inhibition is antagonised by GA. As noted, GAI expression in a plant may affect one or more other characteristics of the plant. A preferred characteristic that may be shared with the Arabidopsis thaliana GAI gene is the ability to complement a GAI null mutant phenotype in a plant such as Arabidopsis thaliana, such phenotype being resistance to the dwarfing effect of paclobutrazol.
Some preferred embodiments of polypeptides according to the present invention (encoded by nucleic acid embodiments according to the present invention) include the 17 amino acid sequence which is underlined in FIG. 4 or a contiguous sequence of amino acids residues with at least about 10 residues with similarity or identity with the respective corresponding residue (in terms of position) in 17 amino acids which are underlined in FIG. 4, more preferably, 11, 12, 13, 14, 15, 16 or 17 such residues.
As is well-understood, homology at the amino acid level is generally in terms of amino acid similarity or identity. Similarity allows for xe2x80x9cconservative variationxe2x80x9d, i.e. substitution of one hydrophobic residue such as isoleucine, valine, leucine or methionine for another, or the substitution of one polar residue for another, such as arginine for lysine, glutamic for aspartic acid, or glutamine for asparagine. Similarity may be as defined and determined by the TBLASTN program, of Altschul et al. (1990) J. Mol. Biol. 215: 403-10, which is in standard use in the art. Homology may be over the full-length of the GAI sequence of FIG. 4, or may more preferably be over a contiguous sequence of 17 amino acids, compared with the 17 amino acids underlined in FIG. 4, or a longer sequence, e.g. about 20, 25, 30, 40, 50 or more amino acids, compared with the amino acid sequence of FIG. 4 and preferably including the underlined 17 amino acids.
At the nucleic acid level, homology may be over the full-length or more preferably by comparison with the 51 nucleotide coding sequence within the sequence of FIG. 3 and encoding the 17 amino acid sequence underlined in FIG. 4, or a longer sequence, e.g. about, 60, 70, 80, 90, 100, 120, 150 or more nucleotides and preferably includeing the 51 nucleotide of FIG. 3 which encodes the underlined 17 amino acid sequence of FIG. 4.
Homologues to gai mutants are also provided by the present invention. These may be mutants where the wild-type includes the 17 amino acids underlined in FIG. 4, or a contiguous sequence of 17 amino acids with at least about 10 (more preferably, 11, 12, 13, 14, 15, 16 or 17) which have similarity or identity with the corresponding residue in the 17 amino acid sequence underlined in FIG. 4, but the mutant does not. Nucleic acid encoding such mutant polypeptides may on expression in a plant confer a phenotype which is insensitive or unresponsive to treatment of the plant with GA, that is a mutant phenotype which is not overcome or there is no reversion to wild-type phenotype on treatment of the plant with GA (though there may be some response in the plant on provision or depletion of GA).
A further aspect of the present invention provides a nucleic acid isolate having a nucleotide sequence encoding a polypeptide which includes an amino acid sequence which is a mutant, allele, derivative or variant sequence of the GAI amino acid sequence of the species Arabidopsis thaliana shown in FIG. 4, or is a homologue of another species or a mutant, allele, derivative or variant thereof, wherein said mutant, allele, derivative, variant or homologue differs from the amino acid sequence shown in FIG. 4 by way of insertion, deletion, addition and/or substitution of one or more amino acids, as obtainable by producing transgenic plants by transforming plants which have a GAI null mutant phenotype, which phenotype is resistance to the dwarfing effect of paclobutrazol, with test nucleic acid, causing or allowing expression from test nucleic acid within the transgenic plants, screening the transgenic plants for those exhibiting complementation of the GAI null mutant phenotype to identify test nucleic acid able to complement the GAI null mutant, deleting from nucleic acid so identified as being able to complement the GAI null mutant a nucleotide sequence encoding the 17 amino acid sequence underlined in FIG. 4 or a contiguous 17 amino acid sequence in which at least 10 residues have similarity or identity with the respective amino acid in the corresponding position in the 17 amino acid sequence underlined in FIG. 4, more preferably 11, 12, 13, 14, 15, 16 or 17.
GAI and gai gene homologues may be identified from economically important monocotyledonous crop plants such as wheat, rice and maize. Although genes encoding the same protein in monocotyledonous and dicotyledonous plants show relatively little homology at the nucleotide level, amino acid sequences are conserved.
In public sequence databases we recently identified several EST sequences that were obtained in random sequencing programmes and share homology with GAI. Table 2 gives details, showing that homologous sequences have been found in various species, including Zea Mays (maize), O. Sativa (rice), and Brassica napus (rape). By sequencing, study of expression patterns and examining the effect of altering their expression, GAI gene homologues, carrying out a similar function in other plants, are obtainable. Of course, novel uses and mutants, derivatives and alleles of these sequences are included within the scope of the various aspects of the present invention in the same terms as discussed above for the Arabidopsis thaliana gene.
A cell containing nucleic acid of the present invention represents a further aspect of the invention, particularly a plant cell, or a bacterial cell.
The cell may comprise the nucleic acid encoding the enzyme by virtue of introduction into the cell or an ancestor thereof of the nucleic acid, e.g. by transformation using any suitable technique available to those skilled in the art.
Also according to the invention there is provided a plant cell having incorporated into its genome nucleic acid as disclosed. The present invention also provides a plant comprising such a plant cell.
Also according to the invention there is provided a plant cell having incorporated into its genome a sequence of nucleotides as provided by the present invention, under operative control of a regulatory sequence for control of expression. A further aspect of the present invention provides a method of making such a plant cell involving introduction of a vector comprising the sequence of nucleotides into a plant cell and causing or allowing recombination between the vector and the plant cell genome to introduce the sequence of nucleotides into the genome.
A plant according to the present invention may be one which does not breed true in one or more properties. Plant varieties may be excluded, particularly registrable plant varieties according to Plant Breeders"" Rights. It is noted that a plant need not be considered a xe2x80x9cplant varietyxe2x80x9d simply because it contains stably within its genome a transgene, introduced into a cell of the plant or an ancestor thereof.
In addition to a plant, the present invention provides any clone of such a plant, seed, selfed or hybrid progeny and descendants, and any part of any of these, such as cuttings, seed. The invention provides any plant propagule, that is any part which may be used in reproduction or propagation, sexual or asexual, including cuttings, seed and so on. Also encompassed by the invention is a plant which is a sexually or asexually propagated off-spring, clone or descendant of such a plant, or any part or propagule of said plant, off-spring, clone or descendant.
The invention further provides a method of influencing the characteristics of a plant comprising expression of a heterologous GAI or gai gene sequence (or mutant, allele, derivative or homologue thereof, as discussed) within cells of the plant. The term xe2x80x9cheterologousxe2x80x9dindicates that the gene/sequence of nucleotides in question have been introduced into said cells of the plant, or an ancestor thereof, using genetic engineering, that is to say by human intervention, which may comprise transformation. The gene may be on an extra-genomic vector or incorporated, preferably stably, into the genome. The heterologous gene may replace an endogenous equivalent gene, ie one which normally performs the same or a similar function in control of growth and/or development, or the inserted sequence may be additional to an endogenous gene. An advantage of introduction of a heterologous gene is the ability to place expression of the gene under the control of a promoter of choice, in order to be able to influence gene expression, and therefore growth and/or development of the plant according to preference. Furthermore, mutants and derivatives of the wild-type gene may be used in place of the endogenous gene. The inserted gene may be foreign or exogenous to the host cell, e.g. of another plant species.
The principal characteristic which may be altered using the present invention is growth.
According to the model of the GAI gene as a growth repressor, under-expression of the gene may be used to promote growth, at least in plants which have only one endogenous gene conferring GAI function (not for example Arabidopsis which has endogenous homologues which would compensate). This may involve use of anti-sense or sense regulation. Taller plants may be made by knocking out GAI or the relevant homologous gene in the plant of interest. Plants may be made which are resistant to compounds which inhibit GA biosynthesis, such as paclobutrazol, for instance to allow use of a GA biosynthesis inhibitor to keep weeds dwarf but let crop plants grow tall.
Over-expression of a GAI gene may lead to a dwarf plant which is correctable by treatment with GA, as predicted by the GAI repression model.
Since gai mutant genes are dominant on phenotype, they may be used to make GA-insensitive dwarf plants. This may be applied for example to any transformable crop-plant, tree or fruit-tree species. It may provide higher yield/reduced lodging like Rht wheat. In rice this may provide GA-insensitive rice resistant to the Bakane disease, which is a problem in Japan and elsewhere. Dwarf ornamentals may be of value for the horticulture and cut-flower markets. Sequence manipulation may provide for varying degrees of severity of dwarfing, GA-insensitive phenotype, allowing tailoring of the degree of severity to the needs of each crop-plant or the wishes of the manipulator. Over-expression of gai-mutant sequences is potentially the most useful.
A second characteristic that may be altered is plant development, for instance flowering. In some plants, and in certain environmental conditions, a GA signal is required for floral induction. For example, GA-deficient mutant Arabidopsis plants grown under short day conditions will do not flower unless treated with GA: these plants do flower normally when grown under long day conditions. Arabidopsis gai mutant plants show delayed flowering under short day conditions: severe mutants may not flower at all. Thus, for instance by GAI or gai gene expression or over-expression, plants may be produced which remain vegetative until given GA treatment to induce flowering. This may be useful in horticultural contexts or for spinach, lettuce and other crops where suppression of bolting is desirable.
The nucleic acid according to the invention may be placed under the control of an externally inducible gene promoter to place the GAI or gai coding sequence under the control of the user.
The term xe2x80x9cinduciblexe2x80x9d as applied to a promoter is well understood by those skilled in the art. In essence, expression under the control of an inducible promoter is xe2x80x9cswitched onxe2x80x9d or increased in response to an applied stimulus. The nature of the stimulus varies between promoters. Some inducible promoters cause little or undetectable levels of expression (or no expression) in the absence of the appropriate stimulus. Other inducible promoters cause detectable constitutive expression in the absence of the stimulus. Whatever the level of expression is in the absence of the stimulus, expression from any inducible promoter is increased in the presence of the correct stimulus. The preferable situation is where the level of expression increases upon application of the relevant stimulus by an amount effective to alter a phenotypic characteristic. Thus an inducible (or xe2x80x9cswitchablexe2x80x9d) promoter may be used which causes a basic level of expression in the absence of the stimulus which level is too low to bring about a desired phenotype (and may in fact be zero). Upon application of the stimulus, expression is increased (or switched on) to a level which brings about the desired phenotype.
Suitable promoters include the Cauliflower Mosaic Virus 35S (CaMV 35S) gene promoter that is expressed at a high level in virtually all plant tissues (Benfey et al, 1990a and 1990b); the maize glutathione-S-transferase isoform II (GST-II-27) gene promoter which is activated in response to application of exogenous safener (WO93/01294, ICI Ltd); the cauliflower meri 5 promoter that is expressed in the vegetative apical meristem as well as several well localised positions in the plant body, eg inner phloem, flower primordia, branching points in root and shoot (Medford, 1992; Medford et al, 1991) and the Arabidopsis thaliana LEAFY promoter that is expressed very early in flower development (Weigel et al, 1992).
The GST-II-27 gene promoter has been shown to be induced by certain chemical compounds which can be applied to growing plants. The promoter is functional in both monocotyledons and dicotyledons. It can therefore be used to control gene expression in a variety of genetically modified plants, including field crops such as canola, sunflower, tobacco, sugarbeet, cotton; cereals such as wheat, barley, rice, maize, sorghum; fruit such as tomatoes, mangoes, peaches, apples, pears, strawberries, bananas, and melons; and vegetables such as carrot, lettuce, cabbage and onion. The GST-II-27 promoter is also suitable for use in a variety of tissues, including roots, leaves, stems and reproductive tissues.
Accordingly, the present invention provides in a further aspect a gene construct comprising an inducible promoter operatively linked to a nucleotide sequence provided by the present invention, such as the GAI gene of Arabidopsis thaliana, a homologue from another plant species or any mutant, derivative or allele thereof. This enables control of expression of the gene. The invention also provides plants transformed with said gene construct and methods comprising introduction of such a construct into a plant cell and/or induction of expression of a construct within a plant cell, by application of a suitable stimulus, an effective exogenous inducer. The promoter may be the GST-II-27 gene promoter or any other inducible plant promoter.
When introducing a chosen gene construct into a cell, certain considerations must be taken into account, well known to those skilled in the art. The nucleic acid to be inserted should be assembled within a construct which contains effective regulatory elements which will drive transcription. There must be available a method of transporting the construct into the cell. Once the construct is within the cell membrane, integration into the endogenous chromosomal material either will or will not occur. Finally, as far as plants are concerned the target cell type must be such that cells can be regenerated into whole plants.
Selectable genetic markers may be used consisting of chimaeric genes that confer selectable phenotypes such as resistance to antibiotics such as kanamycin, hygromycin, phosphinotricin, chlorsulfuron, methotrexate, gentamycin, spectinomycin, imidazolinones and glyphosate.
An aspect of the present invention is the use of nucleic acid according to the invention in the production of a transgenic plant.
A further aspect provides a method including introducing the nucleic acid into a plant cell and causing or allowing incorporation of the nucleic acid into the genome of the cell.
Any appropriate method of plant transformation may be used to generate plant cells comprising nucleic acid in accordance with the present invention. Following transformation, plants may be regenerated from transformed plant cells and tissue.
Successfully transformed cells and/or plants, i.e. with the construct incorporated into their genome, may be selected following introduction of the nucleic acid into plant cells, optionally followed by regeneration into a plant, e.g. using one or more marker genes such as antibiotic resistance (see above).
Plants transformed with the DNA segment containing the sequence may be produced by standard techniques which are already known for the genetic manipulation of plants. DNA can be transformed into plant cells using any suitable technology, such as a disarmed Ti-plasmid vector carried by Agrobacterium exploiting its natural gene transfer ability (EP-A-270355, EP-A-0116718, NAR 12(22) 8711-87215 1984), particle or microprojectile bombardment (U.S. Pat. No. 5,100,792, EP-A-444882, EP-A-434616) microinjection (WO 92/09696, WO 94/00583, EP 331083, EP 175966, Green et al. (1987) Plant Tissue and Cell Culture, Academic Press), electroporation (EP 290395, WO 8706614 Gelvin Debeyserxe2x80x94see attached) other forms of direct DNA uptake (DE 4005152, WO 9012096, U.S. Pat. No. 4,684,611), liposome mediated DNA uptake (e.g. Freeman et al. Plant Cell Physiol. 29: 1353 (1984)), or the vortexing method (e.g. Kindle, PNAS U.S.A. 87: 1228 (1990d). Physical methods for the transformation of plant cells are reviewed in Oard, 1991, Biotech. Adv. 9: 1-11.
Agrobacterium transformation is widely used by those skilled in the art to transform dicotyledonous species. Recently, there has been substantial progress towards the routine production of stable, fertile transgenic plants in almost all economically relevant monocot plants (Toriyama, et al. (1988) Bio/Technology 6, 1072-1074; Zhang, et al. (1988) Plant Cell Rep. 7, 379-384; Zhang, et al. (1988) Theor Appl Genet 76, 835-840; Shimamoto, et al. (1989) Nature 338, 274-276; Datta, et al. (1990) Bio/Technology 8, 736-740; Christou, et al. (1991) Bio/Technology 9, 957-962; Peng, et al. (1991) International Rice Research Institute, Manila, Philippines 563-574; Cao, et al. (1992) Plant Cell Rep. 11, 585-591; Li, et al. (1993) Plant Cell Rep. 12, 250-255; Rathore, et al. (1993) Plant Molecular Biology 21, 871-884; Fromm, et al. (1990) Bio/Technology-8, 833-839; Gordon-Kamm, et al. (1990) Plant Cell 2, 603-618; D""Halluin, et al. (1992) Plant Cell 4, 1495-1505; Walters, et al. (1992) Plant Molecular Biology 18, 189-200; Koziel, et al. (1993) Biotechnology 11, 194-200; Vasil, I. K. (1994) Plant Molecular Biology 25, 925-937; Weeks, et al. (1993) Plant Physiology 102, 1077-1084; Somers, et al. (1992) Bio/Technology 10, 1589-1594; WO92/14828). In particular, Agrobacterium mediated transformation is now emerging also as an highly efficient transformation method in monocots (Hiei et al. (1994) The Plant Journal 6, 271-282).
The generation of fertile transgenic plants has been achieved in the cereals rice, maize, wheat, oat, and barley (reviewed in Shimamoto, K. (1994) Current Opinion in Biotechnology 5, 158-162; Vasil, et al. (1992) Bio/Technology 10, 667-674; Vain et al., 1995, Biotechnology Advances 13 (4): 653-671; Vasil, 1996, Nature Biotechnology 14 page 702).
Microprojectile bombardment, electroporation and direct DNA uptake are preferred where Agrobacterium is inefficient or ineffective. Alternatively, a combination of different techniques may be employed to enhance the efficiency of the transformation process, eg bombardment with Agrobacterium coated microparticles (EP-A-486234) or microprojectile bombardment to induce wounding followed by co-cultivation with Agrobacterium (EP-A-486233).
Brassica napus transformation is described in Moloney et al. (1989) Plant Cell Reports 8: 238-242.
Following transformation, a plant may be regenerated, e.g. from single cells, callus tissue or leaf discs, as is standard in the art. Almost any plant can be entirely regenerated from cells, tissues and organs of the plant. Available techniques are reviewd in Vasil et al., Cell Culture and Somatic Cel Genetics of Plants, Vol I, II and III, Laboratory Procedures and Their Applications, Academic Press, 1984, and Weissbach and Weissbach, Methods for Plant Molecular Biology, Academic Press, 1989.
The particular choice of a transformation technology will be determined by its efficiency to transform certain plant species as well as the experience and preference of the person practising the invention with a particular methodology of choice. It will be apparent to the skilled person that the particular choice of a transformation system to introduce nucleic acid into plant cells is not essential to or a limitation of the invention, nor is the choice of technique for plant regeneration.
In the present invention, over-expression may be achieved by introduction of the nucleotide sequence in a sense orientation. Thus, the present invention provides a method of influencing a characteristic of a plant, the method comprising causing or allowing expression of nucleic acid according to the invention from that nucleic acid within cells of the plant.
Under-expression of the gene product polypeptide may be achieved using anti-sense technology or xe2x80x9csense regulationxe2x80x9d. The use of anti-sense genes or partial gene sequences to down-regulate gene expression is now well-established. DNA is placed under the control of a promoter such that transcription of the xe2x80x9canti-sensexe2x80x9d strand of the DNA yields RNA which is complementary to normal mRNA transcribed from the xe2x80x9csensexe2x80x9d strand of the target gene. For double-stranded DNA this is achieved by placing a coding sequence or a fragment thereof in a xe2x80x9creverse orientationxe2x80x9d under the control of a promoter. The complementary anti-sense RNA sequence is thought then to bind with mRNA to form a duplex, inhibiting translation of the endogenous mRNA from the target gene into protein. Whether or not this is the actual mode of action is still uncertain. However, it is established fact that the technique works. See, for example, Rothstein et al, 1987; Smith et al, (1988) Nature 334, 724-726; Zhang et al, (1992) The Plant Cell 4, 1575-1588, English et al., (1996) The Plant Cell 8, 179-188. Antisense technology is also reviewed in reviewed in Bourque, (1995), Plant Science 105, 125-149, and Flavell, (1994) PNAS USA 91, 3490-3496.
The complete sequence corresponding to the coding sequence in reverse orientation need not be used. For example fragments of sufficient length may be used. It is a routine matter for the person skilled in the art to screen fragments of various sizes and from various parts of the coding sequence to optimise the level of anti-sense inhibition. It may be advantageous to include the initiating methionine ATG codon, and perhaps one or more nucleotides upstream of the initiating codon. A further possibility is to target a regulatory sequence of a gene, e.g. a sequence that is characteristic of one or more genes in one or more pathogens against which resistance is desired. A suitable fragment may have at least about 14-23 nucleotides, e.g. about 15, 16 or 17, or more, at least about 25, at least about 30, at least about 40, at least about 50, or more. Such fragments in the sense orientation may be used in co-suppression (see below).
Total complementarity of sequence is not essential, though may be preferred. One or more nucleotides may differ in the anti-sense construct from the target gene. It may be preferred for there to be sufficient homology for the respective anti-sense and sense RNA molecules to hybridise, particularly under the conditions existing in a plant cell.
Thus, the present invention also provides a method of influencing a characteristic of a plant, the method comprising causing or allowing anti-sense transcription from nucleic acid according to the invention within cells of the plant.
When additional copies of the target gene are inserted in sense, that is the same, orientation as the target gene, a range of phenotypes is produced which includes individuals where over-expression occurs and some where under-expression of protein from the target gene occurs. When the inserted gene is only part of the endogenous gene the number of under-expressing individuals in the transgenic population increases. The mechanism by which sense regulation occurs, particularly down-regulation, is not well-understood. However, this technique is also well-reported in scientific and patent literature and is used routinely for gene control. See, for example, See, for example, van der Krol et al., (1990) The Plant Cell 2, 291-299; Napoli et al., (1990) The Plant Cell 2, 279-289; Zhang et al., (1992) The Plant Cell 4, 1575-1588, and U.S. Pat. No. 5,231,020.
Thus, the present invention also provides a method of influencing a characteristic of a plant, the method comprising causing or allowing expression from nucleic acid according to the invention within cells of the plant. This may be used to influence growth.
Aspects and embodiments of the present invention will now be illustrated, by way of example, with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.